Dont think this is fish related nor water quality but rather light and flow which often is the case. Likely the lighting is your issue and in the future - Any images best under white lighting for proper assessment. I cant tell what type anemone based on darkness of pics but some pointers on anemones- If this is a newer tank, anemones dont do well in tanks under 4 months even up to six months because they are generally sensitive to water changes and require pristine conditions to stay in good shape which often require Good water condition that are stable.
Here are some water parameters to follow.
- Water temp: 77°F - 80°F (stay close to the middle of this range)
- pH level: 8.1 to 8.3
- Alk: 8 to 11 dKH
- Salinity: 1.024 to 1.025
- Nitrate < 5 -10
- Phosphate < .04 - .08
Acclimation is very important for them as well as introduction. Most anemones will move around the tank until they find a suitable spot in the tank to call home. As mentioned, lighting is an important part of their care as they need good light to thrive because they’re photosynthetic which means that they absorb light to produce their energy sources. Anemones have zooxanthellae in their body which they feed on. Without adequate lighting, the anemone will expel the zooxanthellae which many think is poop and turn white which is bleaching and often leads to their death.
A moderate amount of flow is recommended as well as moderate light. Too much flow will cause the anemone to stretch and look shrink making it stringy in appearance. Avoid directing your flow directly at the anemone which stresses them and forces them to move. Assure your light is moderate and flow is not excessive in your case
